HYBRIDIZATION IN THE GENUS GLYCINE SUBGENUS GLYCINE WILLD. (LEGUMINOSAE,PAPILIONOIDEAE)

The genus Glycine is composed of two subgenera, Glycine and Soja. Soja includes the cultivated soybean, G. max, and its wild annual counterpart G. soja, while Glycine includes seven wild perennial species. Hybridization was carried out within and between wild perennial species of the subgenus Glycine. The success rate (pods set/flowers crossed) was 11% for intraspecific and 8% for interspecific crosses. A total of 220 F₁ hybrids was examined morphologically and cytologically where possible. Hybrids within G. canescens (2n = 40) and G. latifolia (2n = 40) were fertile as expected. Glycine clandestina (2n = 40) was morphologically separable into at least three groups, which produced fertile hybrids within each group. One cross between two groups gave vegetatively vigorous but sterile hybrids. The majority of crosses within G. tabacina (2n = 80) were fertile, except that extremely narrow-leaved forms gave sterile hybrids in combination with more usual forms. Sterility was also encountered in G. tomentella when aneuploids (2n = 78) from New South Wales, Australia, were crossed with tetraploids (2n = 80) from either Queensland, Australia, or Taiwan; crosses between the latter two populations resulted in seedling lethality. Cytological behavior of sterile hybrids followed a similar pattern, whether at the diploid or tetraploid level. The frequency of chromosome pairing was approximately half that expected if genomes showed full pairing homology. Bivalent disjunction at anaphase I was usually followed by precocious division of the majority of univalents. Telophase I and II were characterized by lagging chromosomes and micronuclei, so that resulting pollen was misshapen and sterile. Chromosome pairing data from sterile intraspecific hybrids at the tetraploid level may indicate a polyphyletic origin of tetraploids, whereby different diploid populations were involved in their formation. Similarly, chromosome pairing in sterile intraspecific diploid hybrids may indicate that the various diploid groups arose independently of one another. Both 40- and 80-chromosome forms are fully diploidized, however, and if they are of ancient origin, divergence since that time could have resulted in the chromosomal differentiation which becomes apparent when intraspecific hybridization is effected. Diploid (2n = 40) interspecific hybrids G. falcata × G. canescens, and G. falcata × G. tomentella grew poorly and did not reach flowering stage. Diploid (2n = 40) crosses between G. latifolia and G. tomentella produced inviable seedlings. Tetraploid (2n = 80) hybrids between G. tomentella and G. tabacina were vegetatively vigorous but sterile owing to low chromosome pairing at meiosis, indicating little pairing homology between the two species. Diploid hybrids between G. canescens and G. clandestina, however, showed almost complete chromosome pairing at diakinesis and partial fertility. Although morphologically distinct, these two species have not diverged sufficiently to prevent hybridization and possible gene exchange through recombination. Self compatibility, perennial growth habit, and geographic isolation have favored divergence among Glycine populations to the point that gene exchange appears no longer possible in many cases. Internal isolating mechanisms have been shown to operate at various levels of plant development from hybrid lethality at seedling stage, to failure of seed-set in sterile but vegetatively vigorous hybrids.     

CROSSING RELATIONSHIPS AMONG DIPLOID SPECIES OF THE SOLANUM NIGRUM COMPLEX IN NORTH AMERICA

Crosses were made in all combinations of the six diploid species of the Solarium nigrum complex that occur in North America. Some interspecific pollinations failed to yield viable seed; successful crosses gave rise to moderately to highly sterile F₁ hybrids. Results of interspecific crosses suggested phylogenetic relationships that were not completely in accord with those suggested by morphology. Interspecific crosses also gave varied results. All interpopulational crosses within S. interius and S. sarachoides produced fully fertile hybrids. In contrast, hybrids within S. americanum and S. douglasii varied from fully fertile to almost completely sterile. Populations of S. pseudogracile could be divided into two groups which are geographically separated but not morphologically differentiated. Fully fertile hybrids resulted from crosses within a group, whereas crosses between groups gave hybrids with reduced fertility. Four crossing groups were observed within 5. nodiflorum; three of the parental groups are sympatric and are morphologically differentiated. Although hybrid sterility in interspecific crosses is sometimes used to support delimitation of species, the presence of sterility in intraspecific crosses suggests that such an interpretation is unwarranted for the S. nigrum complex. Hybrid sterility, therefore, is not considered to have special taxonomic significance in this complex.     

Sexual reproduction, hybridization, apomixis, and polyploidization in the genus Boechera (Brassicaceae)

Of the 340 genera in the Brassicaceae, apomictic reproduction is found only in the North American genus Boechera. We investigated phylogenetic relationships, ability to hybridize, mating system, and ploidy levels of 92 lines sampled from 85 populations and representing 19 Boechera species. Phylogenetic analyses based on chloroplast DNA sequences identified three lineages in the genus. Reciprocal crosses of each line were made to a common sexual diploid B. stricta tester. The resulting F(1) progeny were analyzed for the inheritance of polymorphic microsatellite loci, genome size, and seed production. Intraspecific B. stricta crosses confirmed that this species is mostly diploid and sexual. Interspecific crosses revealed many other species were diploid and sexual and could be successfully hybridized with the tester. We also found obligate and facultative apomictic diploid and triploid lines. De novo F(1) polyploids (either triploids or tetraploids) were derived from the union of nonreduced (from an apomictic parent) and reduced (from the tester) gametes. However, seed production of these F(1) plants was generally low, suggesting a failure in the transmission of apomixis. The creation of a wide array of segregating genetic populations will facilitate future research on the evolution and inheritance of quantitative variation in Boechera.     

The effect of interplant distance on mating success in federally threatened, self-incompatible Hymenoxys herbacea = H. acaulis var. glabra (Asteraceae)

We investigated large- and fine-scale effects of interplant distance on compatibility, seed set, and seed germination in a rare, self-incompatible perennial, Lakeside daisy (Hymenoxys herbacea = H. acaulis var. glabra). Plants were collected at the Marblehead Peninsula. Ohio, and transplanted to a greenhouse where they were hand-pollinated. For the large-scale analysis, 110 crosses were classified in three categories: Near crosses (0.75-6.70 m), Far crosses (17-72 m), and Very Far crosses (>900 ml. There was no significant effect of interplant distance on compatibility, seed set, or seed germination in these crosses. For the fine-scale analysis, we made 44 crosses with interplant distances ranging from 0.75 to 10 m. At this scale, interplant distance explained 10.9% of the variance of the seed/floret ratio, suggesting that local genetic structure may result in a modest amount of biparental inbreeding. We found no fine-scale effects of interplant distance on compatibility or percentage of seed germination, but it is possible that biparental inbreeding could affect later stages of the life cycle not included in this study. For all distance classes, >80% of the crosses were compatible, indicating that lack of compatibility between mates is not likely to limit seed production. Apparently, presumed population bottlenecks have not been severe enough for genetic drift to eliminate substantial numbers of self-incompatibility alleles.     

红蓝石蒜和中国石蒜的种间杂交

<正> 一、引言 石蒜属Lycoris Herb.植物是园林绿化中的一类颇有开发前景的夏季球根花卉,不仅花色丰富多采,使盛夏少花季节的庭园景色面目一新,而且生长势强,既耐干、涝和贫瘠的土壤,又少病虫害。本文继换锦花L.sprengeri Comes ex Spreng,()×中国石蒜L.     

The basis of natural and artificial postzygotic hybridization barriers in Arabidopsis species

The success or failure of interspecific crosses is vital to evolution and to agriculture, but much remains to be learned about the nature of hybridization barriers. Several mechanisms have been proposed to explain postzygotic barriers, including negative interactions between diverged sequences, global genome rearrangements, and widespread epigenetic reprogramming. Another explanation is imbalance of paternally and maternally imprinted genes in the endosperm. Interspecific crosses between diploid Arabidopsis thaliana as the seed parent and tetraploid Arabidopsis arenosa as the pollen parent produced seeds that aborted with the same paternal excess endosperm phenotype seen in crosses between diploid and hexaploid A. thaliana. Doubling maternal ploidy restored seed viability and normal endosperm morphology. However, substituting a hypomethylated tetraploid A. thaliana seed parent reestablished the hybridization barrier by causing seed abortion and a lethal paternal excess phenotype. We conclude from these findings that the dominant cause of seed abortion in the diploid A. thaliana x tetraploid A. arenosa cross is parental genomic imbalance. Our results also demonstrate that manipulation of DNA methylation can be sufficient to erect hybridization barriers, offering a potential mechanism for speciation and a means of controlling gene flow between species.     

INTERSPECIFIC CROSSES IN KALMIA

Interspecific crosses among Kalmia species are reported for the first time. All possible crosses between K. angustifolia, K. cuneata, K. latifolia, K. hirsuta, K. polifolia var. rosmarinifolia and K. p. var. microphylla were attempted. Several crosses yielded no hybrids, many produced albino or yellow-green inviable seedlings, and six combinations produced at least a few seedlings with near-normal growth habit. Stylar inhibition of pollen tube growth prevented some species from hybridizing; reciprocal differences in crossability and pollen tube growth were noted. Kalmia species are highly heterozygous, but no evidence was found within a species for geographic variation of combining ability except for the two varieties of K. polifolia which had different crossing patterns with the other species. The anthocyaninless form of K. angustifolia, in contrast to the normal, has unique combining ability with K. latifolia. No genetic evidence was found to support the placement of K. hirsuta in another genus (Kalmiella) or to designate K. angustifolia var. caroliniana as a distinct species. Use of all the species to breed improved ornamentals is limited by genetic barriers and sterility of F₁ hybrids.     

Cytogenetics of albinism in polecats of the genus Putorius (Carnivora, Mustelidae)

Karyotypes of two ferret species, Putorius eversmanni (2n = 38) and P. putorius (2n = 40), differ in a single Robertsonian rearrangement, resulting in decreased chromosome number in P. eversmanni. Interspecific hybrids from crosses between P. eversmanni and domesticated albino ferret P. putorius furo are fertile. Hybrid individuals have a chromosome set 2 = 39. Analysis of offspring from crosses of these hybrids showed that albinism is a recessive character. Offspring obtained from crosses between interspecific hybrid ferrets and between these hybrids and parental species was subjected to chromosome analysis. A statistically significant correlation was observed between the chromosome number and albinism. As a rule, albino hybrids have a chromosome number 2n = 40. This suggests that the locus, controlling albinism, is located at one of the two pairs of acrocentric chromsomes involved in Robertsonian rearrangement. The rare occurrence of albino hybrids with 2n = 39 may be attributed to recombination. According to our data, recombination probability is less than 0.5 which indicates that the albinism locus located near the centromere. Possible extrapolation of the data obtained for ferrets to other Mustelidae is discussed.     

Influence of Zinnia angustifolia HBK genotype on embryonic and vegetative development of Z. angustifolia x Z. elegans Jacq. interspecific hybrids

Summary Interspecific crosses between Zinnia angustifolia clones (maternal parents) and Z. elegans lines (paternal parents) were performed to investigate postzygotic barriers among Z. angustifolia X Z. elegans hybrids and to determine influence of parental genotype on embryonic and vegetative development of interspecific hybrids. Variation in percentage of emerged seedlings (PES) and percentage of morphologically normal hybrids (PNH) was attributable to Z. angustifolia clones with minor or no effect attributable to Z. elegans lines. Heterogeneity in PES values among Z. angustifolia clones was due to differences in amount of hybrid embryo breakdown and ungerminable seed. Cytological observations of normal and abnormal interspecific hybrids revealed similar chromosome numbers (2n=23) but indicated a low mitotic index for abnormal hybrids. Genetic analysis of PES and PNH suggested control by multiple genes inherited from the Z. angustifolia genome. Adequate sampling of the Z. angustifolia gene pool would permit exploitation of genetic variability present within the species and allow improvements in PES and PNH for interspecific hybrids.Scientific Article No. A-4479, Contribution No. 7472 of the Maryland Agricultural Experiment Station, Department of Horticulture     

POLYGENIC INHERITANCE OF A BEHAVIORAL PHENOTYPE: INTERSPECIFIC GENETICS OF SONG IN THE HAWAIIAN CRICKET GENUS LAUPALA

The Hawaiian cricket genus Laupala (family Gryllidae) is one of several native genera of flightless crickets found in rain-forest habitat across the Hawaiian archipelago. Species in this genus are morphologically quite similar, but the songs produced by adult males are acoustically distinct. I examined the nature of song variation found within Laupala paranigra and between Laupala kohalensis and L. paranigra, both endemic to the island of Hawaii. Variation within and among species was most notable in the temporal structure of the song, as quantified by the pulse rate. The variation in pulse rate present in natural populations of L. paranigra bred true through the F₁ laboratory generation, suggesting that the intraspecific variation in this species has a genetic basis. Interspecific hybridizations between L. kohalensis and L. paranigra successfully produced F₁, F₂, and backcross generations. Hybrid F₁ males from reciprocal crosses sang with significantly different pulse rates, implicating an X chromosomal contribution to the phenotypic difference between these species. Interspecific patterns of inheritance are most consistent with a type-I genetic architecture. Polygenic inheritance of the interspecific pulse-rate variation was observed, and approximately eight genetic factors were estimated to underlie the difference in pulse rate between L. kohalensis and L. paranigra.     

Effect of three pollination methods on embryo development and seedset in intra- and interspecific crosses between seven Lilium species

Summary Interspecific crosses were made between seven Lilium species, viz. L. candidum, L. concolor, L. dauricum, L. henryi, L. longiflorum, L. nepalense and L. rubellum. A complete diallel cross was carried out between these seven species, including self- and intraspecific pollinations using three pollination methods: normal pollination on the stigma, pollination on the ovary after cutting the style, and pollination on the stigma with the aid of mentor (non-functional, compatible) pollen. Embryo rescue, starting 35 days after pollination, was applied to all interspecific combinations. The percentage of successful crosses was about 2.8% after normal pollination, 5.4% after cut-style pollination and 3.8% with the mentor pollen technique. Crosses with L. nepalense were exceptional in that embryos died during the embryo culture phase. Seventeen cross combinations (including 4 reciprocals) yielded 62 embryo plantlets from 839 interspecific pollinations.     

Interspecific hybridization between Vigna radiata (L.) Wilczek and V. glabrescens

Summary Interspecific hybrids of the mungbean, Vigna radiata (L.) Wilczek (2n=22) and V. glabrescens (2n=44) were generated with the aid of embryo culture. V. glabrescens x V. radiata hybrids were recovered via germination of the immature embryos. Reciprocal hybrids were obtained via shoot formation from embryonic callus. The authenticity of the hybrids was determined by morphological characteristics, chromosome number, and isozyme patterns. The hybrids were highly sterile upon selfing, but backcrossing to the diploid parent yielded viable seeds. Some of the plants resembled the diploid parent morphologically while others resembled neither parent. The backcross plants were sufficiently fertile to give a large number of mature, selfed seeds. Plants obtained differed morphologically and in their isozyme patterns from either parent, indicating introgression. These progeny populations will be used as bridging materials to transfer pest resistance from the wild tetraploid to the cultivated mungbean.     

Quantitative and qualitative differences in morphological traits revealed between diploid Fragaria species

BACKGROUND AND AIMS: The aims of this investigation were to highlight the qualitative and quantitative diversity apparent between nine diploid Fragaria species and produce interspecific populations segregating for a large number of morphological characters suitable for quantitative trait loci analysis. METHODS: A qualitative comparison of eight described diploid Fragaria species was performed and measurements were taken of 23 morphological traits from 19 accessions including eight described species and one previously undescribed species. A principal components analysis was performed on 14 mathematically unrelated traits from these accessions, which partitioned the species accessions into distinct morphological groups. Interspecific crosses were performed with accessions of species that displayed significant quantitative divergence and, from these, populations that should segregate for a range of quantitative traits were raised. KEY RESULTS: Significant differences between species were observed for all 23 morphological traits quantified and three distinct groups of species accessions were observed after the principal components analysis. Interspecific crosses were performed between these groups, and F2 and backcross populations were raised that should segregate for a range of morphological characters. In addition, the study highlighted a number of distinctive morphological characters in many of the species studied. CONCLUSIONS: Diploid Fragaria species are morphologically diverse, yet remain highly interfertile, making the group an ideal model for the study of the genetic basis of phenotypic differences between species through map-based investigation using quantitative trait loci. The segregating interspecific populations raised will be ideal for such investigations and could also provide insights into the nature and extent of genome evolution within this group.     

Maternal–foetal genomic conflict and speciation: no evidence for hybrid placental dysplasia in crosses between two house mouse subspecies

Interspecific hybridization between closely related mammalian species, including various species of the genus Mus, is commonly associated with abnormal growth of the placenta and hybrid foetuses, a phenomenon known as hybrid placental dysplasia (HPD). The role of HPD in speciation is anticipated but still poorly understood. Here, we studied placental and foetal growth in F₁ crosses between four inbred mouse strains derived from two house mouse subspecies, Mus musculus musculus and Mus musculus domesticus. These subspecies are in the early stage of speciation and still hybridize in nature. In accordance with the maternal–foetal genomic conflict hypothesis, we found different parental influences on placental and foetal development, with placental weight most affected by the father's body weight and foetal weight by the mother's body weight. After removing the effects of parents’ body weight, we did not find any significant differences in foetal or placental weights between intra‐subspecific and inter‐subspecific F₁ crosses. Nevertheless, we found that the variability in placental weight in inter‐subspecific crosses is linked to the X chromosome, similarly as for HPD in interspecific mouse crosses. Our results suggest that maternal–foetal genomic conflict occurs in the house mouse system, but has not yet diverged sufficiently to cause abnormalities in placental and foetal growth in inter‐subspecific crosses. HPD is thus unlikely to contribute to speciation in the house mouse system. However, we cannot rule out that it might have contributed to other speciation events in the genus Mus, where differences in the levels of polyandry exist between the species.     

Intergeneric crosses of Psathyrostachys huashanica with Elymus spp. and cytogenetic studies of the hybrids with E. tsukushiensis (Poaceae, Triticeae)

Intergeneric crosses have been made between Psathyrostachys huashanica (2x= 14, NN) and three Elymus species, namely, E. pendulinus (2n = 4x = 28, SSYY), E. ciliaris (2n = 4x = 28, SSYY), and E. tsukushiensis (2n = 6x = 42, SSHHYY). Three accessions of E. tsukushiensis from different localities crossed with P. huashanica produced adult hybrid plants. Although completely sterile, the hybrid plants developed rather vigorously, and were morphologically intermediate between the two parents. Chromosome configuration of 24.62I + 1.51II + 0.03III per cell characterized meioses of the hybrids, but there existed slight variations among different combinations. These results indicate that there is little or no chromosome homoeology between "N" genome of P. huashanica and "S", "H" or "Y" genomes of E. tsukushiensis.     

Intergeneric hybridization between Taeniatherum and different genera of Triticeae, Poaceae

Intergeneric crosses were carried out between the 3 subspecies of Taeniatherum caput-medusae (2x) and 30 different species (33 different cytotypes) representing 11 genera of the Triticeae. Seed set was obtained in 15 combinations, but only 6 of them resulted in adult plants. In the combination with Hordeum bulbosum (2x), a haploid of T. caput-medusae was obtained as a result of selective elimination of the H. bulbosum chromosomes. All other hybrid plants were morphologically intermediate between the parents. They were highly sterile with low pollen fertility caused by very low chromosome pairing at meiosis and without any seed set.
The few hybrid plants obtained and the lack of pairing at meiosis support that Taeniatherum is a distinct genus within the Triticeae.     

Mating interactions between coexisting dipoloid, triploid and tetraploid cytotypes ofHieracium Echioides (Asteraceae)

Experimental crosses between diploids, triploids and tetraploids ofHieracium echioides were made to examine mating interactions. Specifically, cytotype diversity in progeny from experimental crosses, intercytotype pollen competition as a reproductive barrier between diploids and tetraploids, and differences in seed set between intra- and intercytotype crosses were studied. Only diploids were found in progeny from 2x × 2x crosses. The other types of crosses yielded more than one cytotype in progeny, but one cytotype predominated in each cross type: diploids (92%) in 2x × 3x crosses, tetraploids (88%) in 3x × 2x crosses, triploids (96%) in 2x × 4x crosses, triploids (90%) in 4x × 2x crosses, tetraploids (60%) in 3x × 3x crosses, pentaploids (56%) in 3x × 4x crosses, triploids (80%) in 4x × 3x crosses and tetraploids (88%) in 4x × 4x crosses. No aneuploids have been detected among karyologically analyzed plants. Unreduced egg cell production was detected in triploids and tetraploids, but formation of unreduced pollen was recorded only in two cases in triploids. Triploid plants produced x, 2x and 3x gametes: in male gametes x (92%) gametes predominated whereas in female gametes 3x (88%) gametes predominated. Cytotype diversity in progeny from crosses where diploids and tetraploids were pollinated by mixture of pollen from diploid and tetraploid plants suggested intercytotype pollen competition to serve as a prezygotic reproductive barrier. No statistically significant difference in seed set obtained from intra- and intercytotype crosses between diploids and tetraploids was observed, suggesting the absence of postzygotic reproductive barriers among cytotypes.     

DNA barcodes effectively identify the morphologically similar Common Opossum (Didelphis marsupialis) and Virginia Opossum (Didelphis virginiana) from areas of sympatry in Mexico

Two morphologically similar species of opossum from the genus Didelphis-Didelphis virginiana and Didelphis marsupialis-cooccur sympatrically in Mexico. High intraspecific variation complicates their morphological discrimination, under both field and museum conditions. This study aims to evaluate the utility and reliability of using DNA barcodes (short standardized genome fragments used for DNA-based identification) to distinguish these two species. Sequences of the cytochrome c oxidase subunit I (Cox1) mitochondrial gene were obtained from 12 D. marsupialis and 29 D. virginiana individuals and were compared using the neighbor-joining (NJ) algorithm with Kimura's two-parameter (K2P) model of nucleotide substitution. Average K2P distances were 1.56% within D. virginiana and 1.65% in D. marsupialis. Interspecific distances between D. virginiana and D. marsupialis varied from 7.8 to 9.3% and their barcode sequences formed distinct non-overlapping clusters on NJ trees. All sympatric specimens of both species were effectively discriminated, confirming the utility of Cox1 barcoding as a tool for taxonomic identification of these morphologically similar taxa.     

Unidirectional hybridization and reproductive barriers between two heterostylous primrose species in north-west Yunnan,China

Background and Aims

Heteromorphy in flowers has a profound effect on breeding patterns within a species, but little is known about how it affects reproductive barriers between species. The heterostylous genus Primula is very diverse in the Himalaya region, but hybrids there have been little researched. This study examines in detail a natural hybrid zone between P. beesiana and P. bulleyana.

Methods

Chloroplast sequencing, AFLP (amplified fragment length polymorphism) markers and morphological comparisons were employed to characterize putative hybrids in the field, using synthetic F₁s from hand pollination as controls. Pollinator visits to parent species and hybrids were observed in the field. Hand pollinations were conducted to compare pollen tube growth, seed production and seed viability for crosses involving different morphs, species and directions of crossing.

Key Results

Molecular data revealed all hybrid derivatives examined to be backcrosses of first or later generations towards P. bulleyana: all had the chloroplast DNA (cpDNA) of this species. Some individuals had morphological traits suggesting they were hybrids, but they were genetically similar to P. bulleyana; they might have been advanced generation backcrosses. Viable F₁s could not be produced with P. bulleyana pollen on P. beesiana females, irrespective of the flower morphs used. Within-morph crosses for each species had very low (<10 %) seed viability, whereas crosses between pin P. bulleyana (female) and pin P. beesiana had a higher seed viability of 30 %. Thus genetic incompatibility mechanisms back up mechanical barriers to within-morph crosses in each species, but are not the same between the two species. The two species share their main pollinators, and pollinators were observed to fly between P. bulleyana and hybrids, suggesting that pollinator behaviour may not be an important isolating factor.

Conclusions

Hybridization is strongly asymmetric, with P. bulleyana the only possible mother and all detected hybrids being backcrosses in this direction. Partial ecological isolation and inhibition of heterospecific pollen, and possibly complete barriers to F₁ formation on P. beesiana, may be enough to make F₁ formation very rare in these species. Therefore, with no F₁ detected, this hybrid zone may have a finite life span as successive generations become more similar to P. bulleyana.     

Tetraploids,triploids, and 2n pollen from diploid interspecific crosses with cassava

Summary Interspecific crosses of five cultivated cassava varieties (2n=36) were made with two related Manihot species, M. epruinosa (2n=36) and M. glaziovii (2n=36). From these diploid interspecific crosses, four spontaneous tetraploids (2n=4x=72) and two triploids (2n=3x=54) were isolated for the first time in cassava. Occurrence of relatively high frequencies (0.1%–35.6%) of 2n pollen and of apomixis seems to be associated with sexual polyploidization. The tetraploids and triploids were very vigorous and one of the tetraploids performed as well as the best variety in uniform yield trials conducted in Nigeria. These spontaneous polypoloids provide greater genetic variation and offer an opportunity to breed radically new cassava varieties. Approaches for isolating and utilizing the polyploid cassava clones for varietal and population improvement are discussed.